USC Dornsife Scientific Diving: The Coconut Crab in Guam

Birgus latro is the largest terrestrial arthropod in the world, in some cases having a leg span of over three feet and weighing over nine pounds. It is more commonly known as the coconut crab, due to its diet. Coconut crabs are mainly scavengers, feeding on various tropical fruits including coconuts. Their two powerful front chelipeds allow them to tear through the tough husk of a coconut and feed on the flesh. In addition, they can climb trees up to 20 feet high just to reach growing coconuts. But these crabs also have other nicknames including “palm thief” or “robber crab.” These names are derived from the crab’s tendencies to steal human possessions, mistaking them for food. During World War II, these clever coconut crabs gained a reputation for stealing items from the trenches of American marines.

The coconut crab is found in abundance on islands throughout the western Pacific and eastern Indian oceans. However, on the island of Guam, the coconut crab has faced a huge decline in numbers. In 1981, Birgus latro was listed as a “vulnerable species” on the IUCN Red List, however the lack of accurate figures has caused the crab to be classified as “data deficient” (Elderedge 1996).

Coconut crab populations across oceanic islands are heavily correlated with the level of human activity. On island nations with larger human populations, coconut crabs are very scarce. On Guam, coconut crab sightings have become very rare. The coconut crab’s population decline on this island can be attributed to two main reasons: human predation and habitat disruption.

Guamanians consider coconut crab meat a delicacy. The local Chamorro people have eaten them for centuries, and the meat holds special significance at important events such as weddings (Fletcher 1993). The local demand for coconut crab meat is so high that the crab population on Guam cannot sustain it alone. Amesbury (1980) reports they were once commonly imported from the Northern Mariana Islands. Their shells also are commonly sold as tourist souvenirs. Unfortunately, these two uses directly compete with one another—because extracting the meat requires cracking the crab’s hard exterior (exoskeleton), the shells cannot be preserved as souvenirs afterward.

Additionally, coconut crabs are threatened by the development of Guam’s coastal zone. They are typically found within a few miles of the ocean, where females have adequate access to the ocean to release their eggs, which hatch only upon contact with saltwater. Recently though, heavy coastal development has modified a large portion of the crab’s preferred habitat. Ultimately, such changes further contribute to the decreasing population of coconut crabs on Guam by reducing the number of breeding individuals (Amesbury 1980).

It is imperative that the coconut crab population on Guam be preserved both for its economic and cultural value. Currently, several measures are in place to protect the crabs: only crabs of a certain size can be collected, and it is illegal to catch any crab carrying eggs. However, more needs to be done to protect these unique and valuable creatures.

At present, the highest crab densities on Guam can be found on military installations, where limited civilian access makes it difficult to hunt crabs (Amesbury 1980). More areas specifically designed to protect the crabs can be created, especially in locations that can protect females and their eggs. Furthermore, establishing moratoriums on crab hunting would allow the dwindling population to regain momentum. A moratorium would also allow larger crabs to flourish, because the larger crabs are targeted first for harvest. While a complete ban on crab hunting is not practical because use of the crab is deeply ingrained in local Chamorro and Guam culture, effective use of these two methods could significantly help reestablish a thriving coconut crab population for the island, and ensure the sustainable relationship between the coconut crab and the people of Guam.

About the Author: Emily Lu is a sophomore in the USC Dana and David Dornsife College of Letters, Arts and Sciences working towards a BS degree in Environmental Studies with a minor in Business. Emily has a strong interest in sustainable business and through her participation in the Guam and Palau course she looks forward to learning more about ecologically sustainable development.

Fletcher, J. (1993). Coconut Crabs. Nearshore Marine Resources of the South Pacific: Information for Fisheries Development and Management. Suva, Fiji: International Centre for Ocean Development. 643-681.

Fisheries and Aquaculture Department of the Food and Agriculture Organization of the United Nations. (1992). Assessment of the status of the coconut crab Birgus latro on Niue Island with recommendations regarding an appropriate resource management strategy. FAO Corporate Document Repository. 72. Retrieved from http://www.fao.org/docrep/field/003/AC281E/AC281E05.htm#ch5.3.1

Editor’s note: Scientific Research Diving at USC Dornsife is offered as part of an experiential summer program offered to undergraduate students of the USC Dana and David Dornsife College of Letters, Arts and Sciences. This course takes place on location at the USC Wrigley Marine Science Center on Catalina Island and throughout Micronesia. Students investigate important environmental issues such as ecologically sustainable development, fisheries management, protected-area planning and assessment, and human health issues. During the course of the program, the student team will dive and collect data to support conservation and management strategies to protect the fragile coral reefs of Guam and Palau in Micronesia.

Instructors for the course include Jim Haw, Director of the Environmental Studies Program in USC Dornsife, Assistant Professor of Environmental Studies David Ginsburg,, SCUBA instructor and volunteer in the USC Scientific Diving Program Tom Carr and USC Dive Safety Officer Gerry Smith of the USC Wrigley Institute for Environmental Studies

The views expressed are those of the author(s) and are not necessarily those of Scientific American.

ABOUT THE AUTHOR(S)

Jim Haw

Dr. Jim Haw is Ray R. Irani Professor of Chemistry and director of the Environmental Studies Program in the USC Dana and Dornsife College of Letters, Arts and Sciences. He is also a scientific, technical and recreational diver.

Scientific American is part of Springer Nature, which owns or has commercial relations with thousands of scientific publications (many of them can be found at www.springernature.com/us). Scientific American maintains a strict policy of editorial independence in reporting developments in science to our readers.